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Screen Tricks

published on 05 Dec 2009, tagged with arch linux screen bash

Hopefully, if you're a CLI junky, you've heard of GNU/screen. And if you've heard of it, chances are you're using it.

Screen is a terminal multiplexer. This means that you can start screen in one terminal (say, your SSH connection) and open any number of terminals inside that terminal. This lets me have mutt, ncmpcpp, and a couple of spare shells all open inside my single PuTTY window at work.

This is a great use of screen, but the benefits don't have to end there. When I'm not at work but at home, I can use screen to run applications which I don't want to end if I want to change terminals, log in and out, or even if all of X comes crashing down around me.

See, screen can detach (default binding: C-a d). Better still, It will auto-detach if the terminal it's in crashes or you logout. You can then re-attach it later, from any other ssh session, tty, or X terminal.

This is great for apps like rtorrent and irssi, it's also great for not losing any work if your ssh connection gets flaky. Just re-connect and re-attach.

So now I have a dilemma. When I'm at work, I want to start screen and get a few fresh tabs set up as I've defined in ~/.screenrc: mutt, ncmpcpp, and three shells. But at home I don't want those things to load, I instead want only rtorrent or only irssi to load up in the new screen window.

Furthermore, if rtorrent or irssi are already running in some detached screen somewhere, I don't want to create an entirely new session, I'd rather grab that one and re-attach it here.

The goal was to achieve this without changing the commands I run day to day, affecting any current keybinds, or using any overly complicated scripts.

So, how do I do this as simply and easily as possible? Environment variables.

How to do it

First we set up one main ~/.screenrc which is always called. Then we set up a series of "screenrc extensions" which only load the apps in the screen session via a stanza of screen -t <name> <command> lines.

Next, we dynamically choose which "screenrc extension" to source from the main ~/.screenrc via two environment variables which are either exported from ~/.bashrc (the default) or explicitly set when running the command (the specialized cases).

So, set up a ~/.screenrc like this:

# screen config file; ~/.screenrc

# put all our main screen settings like
# term, shell, vbell, hardstatus whatever
#
# then add this:

# sources environment-specific apps
source "$SCREEN_CONF_DIR/$SCREEN_CONF"

# you can even add some tabs you'll always
# open no matter what

# then always open some terms
screen -t bash $SHELL
screen -t bash $SHELL
screen -t bash $SHELL

Now, how does screen know what "screenrc extension" to source? By setting those variables up in ~/.bashrc:

# dynamically choose which tabs load in screen
export SCREEN_CONF_DIR="$HOME/.screen/configs"
export SCREEN_CONF="main"

In a clean environment, screen will source that default ~/.screen/configs/main, which will:

# example: screen -t [name] [command]
screen -t mail mutt
screen -t music ncmpcpp

Why is this useful? Because, now I can do something like this:

SCREEN_CONF=rtorrent screen

And screen will instead source that explicitly set ~/.screen/configs/rtorrent which yields:

# example: screen -t [name] [command]
screen -t torrents rtorrent 

Et viola, no mutt or ncmpcpp, but rtorrent instead (same thing happens with irssi).

Oh, but it gets better! Now we'll add some aliases to ~/.bashrc to complete the whole thing:

alias irssi='SCREEN_CONF=irssi screen -S irssi -D -R irssi'
alias rtorrent='SCREEN_CONF=rtorrent screen -S rtorrent -D -R rtorrent'

Oh how beautiful, how simple, how easy. I type rtorrent, what happens?

Screen checks for any running screens with session-name "rtorrent" and re-attaches here and now. If none are found, screen opens a new screen (using the rtorrent file) and names the session "rtorrent" so we can -D -R it explicitly thereafter.

All of this happens for irssi too, and can be used for any app (or multi-app setup) you want.

Pretty KISS if I do say so.